Stem cells are found in all multi cellular organisms. They are characterized by the ability to renew themselves through mitotic cell division and differentiate into a diverse range of specialized cell types. The two broad types of mammalian stem cells are: embryonic stem cells that are isolated from the inner cell mass of blastocysts, and adult stem cells that are found in adult tissues. In a developing embryo, stem cells can differentiate into all of the specialized embryonic tissues. In adult organisms, stem cells and progenitor cells act as a repair system for the body, replenishing specialized cells, but also maintain the normal turnover of regenerative organs, such as blood, skin, or intestinal tissues.
Stem cells can now be grown and transformed into specialized cells with characteristics consistent with cells of various tissues such as muscles or nerves through cell culture. Highly plastic adult stem cells from a variety of sources, including umbilical cord blood and bone marrow, are routinely used in medical therapies. Embryonic cell lines and autologous embryonic stem cells generated through therapeutic cloning have also been proposed as promising candidates for future therapies.
The classical definition of a stem cell is typically indicative of two properties: self-renewal, the ability to go through numerous cycles of cell division while maintaining the undifferentiated state, and potency, the capacity to differentiate into specialized cell types. In some embodiments, stem cells are either totipotent or pluripotent, i.e. they are able to give rise to any mature cell type, although multipotent or unipotent progenitor cells are sometimes referred to as stem cells.
Potency specifies the differentiation potential (the potential to differentiate into different cell types) of the stem cell:
Totipotent stem cells (also known as omnipotent stem cells) can differentiate into embryonic and extraembryonic cell types. These cells can construct a complete, viable, organism. The cells are produced from the fusion of an egg and sperm cell. Cells produced by the first few divisions of the fertilized egg are also totipotent.
Pluripotent stem cells (PSCs) are the descendants of totipotent cells and can differentiate into nearly all cells, i.e., cells derived from any of the three germ layers, including endoderm (interior stomach lining, gastrointestinal tract, the lungs), mesoderm (muscle, bone, blood, urogenital), and ectoderm (epidermal tissues and nervous system).
Multipotent stem cells can differentiate into a number of cells, but only those of a closely related family of cells.
Oligopotent stem cells can differentiate into only a few cells, such as lymphoid or myeloid stem cells.
Unipotent cells can produce only one cell type, their own, but have the property of self-renewal which distinguishes them from non-stem cells (e.g., muscle stem cells).
Embryonic and induced pluripotent stem cells have had an unprecedented impact on our ability to study human diseases and to generate replacement tissues that are therapeutically effective in animal models.
In developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized cell type. Most successful efforts to direct the differentiation of human PSCs into therapeutic cell types have been based on studies of embryonic organ development. Examples include the generation of liver hepatocytes and pancreatic endocrine cells, which have shown functional potential in animal models of liver disease and diabetes. Similarly, differentiation of PSCs into intestine may provide therapeutic benefit for diseases such as necrotizing enterocolitis, inflammatory bowel diseases and short gut syndromes.
As discussed above, a pluripotent stem cell has the potential to differentiate into any of the three germ layers: endoderm (interior stomach lining, gastrointestinal tract, the lungs), mesoderm (muscle, bone, blood, urogenital), and ectoderm (epidermal tissues and nervous system). As such, pluripotent stem cells can give rise to any fetal or adult cell type. However, the fate of the particular pluripotent stem cells is controlled by numerous cellular signaling pathway and numerous factors. Further, the pluripotent stem cells alone cannot develop into a fetal or adult animal because they lack the potential to contribute to extraembryonic tissue, such as the placenta.
What is needed in the art are methods and systems for accurately controlling the destination of the pluripotent stem cells in order to create the specific type of tissue or organism of desire.